In the manufacture of a rubber-fiber composite, there are known methods of bonding a sulfur-containing rubber composition with fibers. These methods include the steps of subjecting the fibers to surface treatment with a resorcin-formalin-rubber latex (RFL), soaking them in a cement solution of the foregoing rubber composition dissolved in an organic solvent, such as toluol (i.e., toluene), methyl ethyl ketone, or n-hexane, and adhering the treated fibers to the rubber composition.
These methods are effective to some extent in enhancing adhesion at ambient temperature (i.e., about 20.degree. C. to 35.degree. C.). However, these methods cannot provide an adhesive strength sufficient to withstand prolonged use in the hot environment (e.g., about 120.degree. C. to about 150.degree. C.) in which rubber-fiber composites are frequently used. These rubber-fiber composites that cannot withstand prolonged use in hot environments are liable to have problems resulting from interlayer peeling.
In making multi-rib belts, raw edge belts, and flat belts, which are used in hot environments such as those that occur around automobile engines, general purpose rubbers, such as chloroprene rubber (CR), have been generally used. However, heat resistant rubbers, such as epichlorohydrin rubber (CHR), chlorosulfonated polyethylene rubber (CSM), and ethylene propylene diene rubber (EPDR), are now used in an attempt to improve durability at engine temperatures which have increased because of exhaust emission controls and increased automobile speeds. But these heat resistant rubbers have a short service life and other unacceptable properties. For example, CHR has poor thermosoftening and low temperature resistance, CSM has the disadvantages of large internal heat generations, and poor cold and oil resistance, and EPDM has insufficient oil resistance.
Under the circumstances, hydrogenated acrylonitrile-butadiene rubber (H-NBR) compositions are attracting much attention because of its excellent oil and heat resistance.
However, incorporation of sulfur into H-NBR, the usual manner for bonding rubber with fibers, has an adverse effect on the heat resistance of the H-NBR. Therefore, in order to retain the desired concurrent heat and oil resistance of the H-NBR, enhanced adhesion between the H-NBR and fibers with a reduced amount of sulfur is necessary.
In Japanese Patent Examination Publication No. 24131/1985, a method of bonding an unvulcanized H-NBR composition with fibers is disclosed which comprised the steps of treating fibers with a rubber latex solution of a hydrogenated acrylonitrile-butadiene latex having a carboxyl group content of 3%, and a mixture of resorcin and formalin prepared so as to give a rubber latex to resorcin-formalin mixture ratio of 10:1 to 2:1 by solid weight and a resorcin to formalin molar ratio of 1:3 to 3:1, and then vulcanizing the H-NBR composition while sticking it to the treated fibers.
While the bonding method proposed in the above-identified Japanese Publication was effective in improving adhesive strength at normal temperatures, it could not provide an adhesive strength sufficient to withstand the above-mentioned high-temperature, hot environment. Moreover, the method had limitations in industrial application since it needed to specify hydrogenated acrylonitrile-butadiene latex as the rubber latex to be used.
The present invention solves at least some of the above problems by providing a new bonding method. The rubber-fiber composite produced by the method can be used to improve the durability of rubber products, including belts, used in a hot environment. The method bonds a rubber composition including mainly hydrogenated acrylonitrile-butadiene rubber (a H-NBR composition) with a percentage of butadiene saturation of about 80 mole % or more with fibers with sufficient adhesion to inhibit interlayer peeling even if the rubber-fiber composite is subjected to thermal deterioration.